Dielectric materials play an extensive role in both industrial applications and scientific research areas. In the modern integrated circuit industry, as electrical components are miniaturized, there are palpable needs for dielectric measurements of low-loss thin materials. The use of fine-line signal conductors requires thinner, possibly laminated, low-dielectric constant printed-wiring board materials. On the other hand, compact antenna arrays require high-dielectric constant substrates to obtain phase shifts. Moreover, lightweight structural composites in air- and space-craft, Kevlar body-armor and ceramic-matrix-composites for thermal stability in hot engine environments are examples of some of the recently developed applications of low-conductivity materials. As a result of these increased applications of dielectrics, the quantitative dielectric property characterization of these dielectric materials becomes markedly important for the process control in manufacturing, optimization of electrical apparatus design and performance, and system monitoring and diagnostics.
A number of high frequency nondestructive evaluation (NDE) techniques have been developed for dielectric measurements with their own specific applications [1]. Transmission-line techniques are capable of measuring material permittivity by an open-circuit termination. The material properties of the test-piece can be interpreted from the reflection coefficient of the system. Open resonators have also been used in measuring low-loss materials in the millimeter wavelength range [2] and a certain open resonator system for measuring anisotropic thin films has been developed and is able to obtain the material tensor permittivity values [3]. Measurements using surface electromagnetic waves are quite applicable for low-loss dielectric thin films and layered substrates, since they possess a high quality factor and are therefore sensitive to loss [4]. Evanescent-field dielectrometry has been utilized in diagnosing and monitoring fresco degradations resulting from moisture and soluble salts [5]. Besides, broadband dielectric measurements (0.01 to 3 GHz) on the effects of exposure of thick film adhesive-bonded structures to moisture have been reported [6], where the data obtained are complemented by mechanical testing and failure analysis of the bond structure measured as a function of the exposure time. However, the focus here is on describing electrostatic and low frequency NDE techniques for dielectric measurements.
One important and practical field of material dielectric property characterization is dielectrometry, which derives the complex permittivity of a test-piece from the measured sensor capacitance. Interdigital dielectrometry sensors, with increased effective length and output capacitance between the electrodes because of their interdigital structure, have been used for dielectrometry measurements for a long time. An excellent review paper on interdigital sensors and transducers is [7], in which the physical principles, sensor design and fabrication, and relevant applications of interdigital sensors are discussed in detail. These interdigital dielectrometry sensors have been applied in many fields such as material property monitoring, humidity and moisture sensing, electrical insulation properties sensing, monitoring of curing processes, chemical sensing, biosensing, and so on. For example, using a secant method root-searching routine for parameter estimation, interdigital electrode dielectrometry has been made capable of measuring the continuum parameters of heterogeneous media [8], which include material thickness, material permittivity with thickness known, and material surface conductivity with thickness known. The optimization of multi-wavelength interdigital dielectrometry instrumentation and algorithms has also been described in [9]. Through variation of geometrical design, materials, manufacturing processes, electronic circuitry, and considerations of accumulated effects of non-ideal geometry of experimental setups, improvement of sensor performance can be achieved. Additionally, design principles for multichannel fringing electric field sensors, especially detailed analysis on how the sensor geometry affects the sensor performance and tradeoffs among different design objectives, have been carried out [10] providing insight into design of capacitive sensors in general.
Apart from using interdigital dielectrometry sensors, other sensor configurations have been used to characterize defects, moisture content, temperature, aging status, delamination, and other inhomogeneities in dielectric materials. For example, rectangular capacitive array sensors have been used for the detection of surface and subsurface features in dielectrics and surface features in conductive materials [11]. Cylindrical geometry quasistatic dielectrometry sensors with signal interpretation based on semi-analytical models have also been developed in recent years to measure the permittivity of a dielectric plate [12]. For water intrusion detection in composite structures, rectangular coplanar capacitance sensors with high sensitivity have been developed [13] on the basis that the presence of defects, such as water, leads to changes of dielectric characteristics in the structure, resulting in variations in the sensor measured capacitance. Using a similar principle, rectangular coplanar capacitance sensors have been applied for damage detection in laminated composite plates [14]. Also, the influence of electrode configurations on a differential capacitive rain sensor, which consists of a sensitive capacitor whose capacitance changes in the presence of water and an insensitive reference capacitor, have been investigated in [15]. Moreover, these capacitance techniques have even been employed for the continuous monitoring of the thickness of biofilms and tissue cultures [16].
Electrical capacitance tomography (ECT) is another capacitance measurement technique that is used to image cross-sections of industrial processes containing dielectric materials [17]. The principle is that through image reconstruction for ECT, the test-piece permittivity distribution and therefore the material distribution over its cross-section can be determined. Over the past decades, research progress on both the hardware design [18, 19] and sensor configuration optimization [20] of ECT systems has been made successfully.
Despite these advances in various capacitance measurement techniques, problems remain. What is needed is a sensor and associated methods and systems which can be used in applications, such as, but not limited to quantitative characterization of material properties of multi-layered structures, detection of water or excessive inhomogeneties in structures such as radome structures, and other applications.